Pneumatic or gas springs are commonly used to provide a counterbalance force for closure units, such as lids, doors and cabinet fronts, and to provide gas spring replacement for mechanical spring. In the automotive field, for example, pneumatic springs are used to assist in opening and supporting trunk lids and hatchbacks. In such applications, the counterbalance spring assemblies are compressed when the lid is closed, and they extend under differential pressure force acting on the piston when the lid is opened.
In conventional pneumatic springs, both the extension and compression chambers are pressurized, and therefore, the pressure differential between the cylinder and the atmosphere only acts on the effective cross-section area of the piston rod which lies in a plane 90.degree. to the longitudinal axis of the rod. As a result, and in a majority of applications, a relatively high internal cylinder pressure is required to cause the spring to extend. For example, in automotive applications it is not uncommon for the pneumatic springs to be pressurized to 2000 psi or more. These high operating pressures impose stringent strength requirements on the materials used to fabricate the pneumatic spring components and this adds to the complexity of the manufacturing process.